Micro-particles are placed between nano-particles on the left and macroscopic particles on the right of the size scale. There are no sharp borders in both directions and the size range is mostly defined between 100 nm and 100 μm. Biological cells, for example, belong to the micro-particles if considered by size. Micro-particles are classified under different aspects: natural and synthetic; organic and inorganic; crystalline and amorphous; porous and non-porous; core-shell-, matrix- or homogeneous structure etc. The manufacturing methods for micro-particles are generally divided into top-down and bottom-up procedures. In the first case a macroscopic material is crashed by milling or high pressure homogenisation, in the second one, the particles are assembles from their molecular compounds by precipitation, crystallization, spray drying etc.
Furthermore, micro-particle assembling procedures mimicking biological processes have been developed in recent years. In generally, the particles are grown by interactive structuring of organic and inorganic materials by a so-called biomimetic process.
Some of the procedures described above allow the synthesis of micro-particles with a special inner structure. Typical examples are porous micro-particles with a vast inner surface due to the pores and cavities. A very interesting fine structure can be generated if the porous particles are used for assembling of complementary structures. The particles are then consisting of the porous template and the structure that fills the pores and cavities.
For a variety of applications, it is desired to produce a high quantity of micro-particles, which should have a well defined mean size. Further, the formed micro-particles should be stable to be stored for a long time. Easy and cost-efficient manufacture of micro-particles comprised of different compounds is also an ongoing desire.